Fix non-deterministic inherit printing

[infinisil]

Motivation

In very rare cases (I had about 7 cases out of 32200 files!), the order of how inherit-from bindings are printed when using nix-instantiate --parse gets messed up.

The cause of this seems to be because the std::map the bindings are placed in is keyed on a pointer, which then uses an implementation-defined strict total order.

The fix here is to key the bindings on their displacement instead, which maintains the same order as they appear in the file.

Unfortunately I wasn't able to make a reproducible test for this in the source, there's something about the local environment that makes it unreproducible for me.

However I was able to make a reproducible test in a Nix build on a Nix version from a very recent master:

nix build github:infinisil/non-det-nix-parsing-repro

which shows the difference (prettified for easy viewing):

diff --git a/a.nix b/b.nix
index 6f0637998..a5fd9ebfa 100644
--- a/a.nix
+++ b/b.nix
@@ -1,6 +1,6 @@
 ({ crun, emptyDirectory, haskell, haskellPackages, lib, makeWrapper, stdenv }: (
-  let inherit (lib) makeBinPath;
-  inherit ((haskell).lib.compose) addBuildTools appendConfigureFlags justStaticExecutables overrideCabal;
+  let inherit ((haskell).lib.compose) addBuildTools appendConfigureFlags justStaticExecutables overrideCabal;
+  inherit (lib) makeBinPath;
   bundledBins = ((lib).optional (stdenv).isLinux crun);
   overrides = (old: {
     hercules-ci-agent = (overrideCabal (o: { buildTarget = "lib:hercules-ci-agent hercules-ci-agent-unit-tests"; configureFlags = ((o).configureFlags or ([ ]) ++ [ (("--bindir=" + emptyDirectory + "/hercules-ci-built-without-binaries/no-bin")) ]); enableSharedExecutables = false; isExecutable = false; isLibrary = true; postInstall = ""; }) (old).hercules-ci-agent);

Context

I've found this when I tried to verify that nix-instantiate --parse doesn't change upon applying a formatter on Nixpkgs. Turns out it does change at least because of this.

Another unrelated instance of this is #11120

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[roberth]

❤️

[roberth]

This could be considered the root cause.
Indexing by a pointer is not necessarily wrong, but enumerating items ordered by addresses is non-deterministic.

[infinisil]

Applied and rebased with your suggestion

[roberth]

Thanks!